1. Field of the Invention
The present invention relates to a cathode assembly used for an electron tube such as a color cathode ray tube and, more particularly, to an improvement in a black coating formed inside the cathode sleeve of the cathode assembly.
2. Description of the Related Art
Recently, a color cathode ray tube having a high resolution, which is achieved by increasing the number of scanning lines, and a display tube compatible with high-frequency signals have been developed. There have been demands for a projection tube and the like to increase their brightness. It is required for tubes suitable for these application purposes to greatly increase the density of electrons emitted from the cathode. A great deal of attention has been paid to an impregnated cathode because of these demands. In general, an impregnated cathode obtains a higher current density than an oxide cathode. Therefore, this impregnated cathode has been used for an electron tube such as a traveling wave tube or a klystron. As an application which effectively uses the high current density characteristics of an impregnated cathode, a color picture tube incorporating the above-mentioned impregnated cathode has recently been developed.
As is well known, the operating temperature of an impregnated cathode is higher than that of an oxide cathode by about 200.degree. C. Accordingly, the heater temperature in the impregnated cathode is high, which reaches 1,250.degree. C. in rated operation conditions. Consequently, thermal distortion of the heater and a deterioration in breakdown voltage performance between the heater and the cathode tend to occur. Several attempts have been made to decrease the heater temperature by increasing the efficiency of heat transfer from the heater to the cathode. For example, an impregnated cathode assembly obtained by forming a black layer containing a refractory metal or a refractory metal powder and an inorganic binder on the inner surface of a cathode sleeve is proposed in Jpn. Pat. Appln. KOKAI Publication No. 61-288339. According to this proposal, for example, a slurry or suspension obtained by adding a mixture of a tungsten powder and an aluminum oxide powder, i.e., an alumina powder, in an alumina sol in which alumina whiskers (0.1 .mu.m.times.0.01 .mu.m) are dispersed in an acetic acid solution, is coated and dried on the inner surface of a tantalum (Ta) sleeve, and the coating is sintered at a temperature of about 1,600.degree. C. for five minutes, thus forming a black layer on the inner surface of the sleeve. In this black layer, the alumina whisker as a binder enters between tungsten grains and alumina grains to increase the bonding strength therebetween.
A cathode sleeve supporting an electron emitting portion is formed to have a very small thickness, e.g., 15 .mu.m to 20 .mu.m, to improve the heat efficiency by suppressing heat conduction to portions other than the electron emitting portion. If a black layer is attached to the inner surface of such a thin cathode sleeve by the above-described method, a great deterioration in strength occurs. As a result, fracture, cracking, or the like tends to occur in the manufacturing process, and deformation of the sleeve tends to occur during an operation owing to thermal fatigue. Especially when a cathode sleeve is made of tantalum (Ta) or a tantalum (Ta) alloy, it is confirmed that a compound is generated in the entire sleeve by a reaction between alumina and the tantalum (Ta) sleeve to cause a great deterioration in strength. If this sleeve is deformed during a cathode operation, the characteristics of the color cathode ray tube, especially the cutoff characteristics, are changed to cause a degradation in brightness or color misregistration. If an alumina sol having a needle-like structure is used, electric fields concentrate on the needle-like tips of alumina grains. As a result, dielectric breakdown tends to occur between the heater and the sleeve.
As a method of attaching a black coating on such a cathode sleeve, for example, a black coating forming method disclosed in Jpn. Appln. KOKOKU Publication No. 52-28631 is known. In this method, a sleeve is dipped in a slurry or suspension of a mixture of a tungsten powder and an alumina powder, and the slurry is dried. The slurry is then sintered to form a black coating. In addition, as disclosed in Jpn. Appln. KOKAI Publication Nos. 61-288339 and 2-72533, another method is known, in which a slurry of a mixture of a tungsten powder and an alumina powder is injected into a sleeve, and an unnecessary slurry is removed by vacuum suction after an elapse of a predetermined period of time. Thereafter, the slurry on the sleeve surface is dried and sintered to form a black coating.
Of the methods of attaching a black coating on a cathode sleeve, the former method of attaching a black layer on a sleeve by dipping the sleeve in a slurry of a black coating material, as disclosed in Jpn. Appln. KOKOKU Publication No. 52-28631, is suitable for the formation of a black coating on both the inner and outer surfaces of a sleeve, but there is a problem that this method is not suitable for the formation of a black coating on only the inner surface of a sleeve. In addition, this makes it difficult to ensure uniformity of the thickness of a coating. In contrast to this, the method of injecting a slurry into a sleeve, and removing an unnecessary slurry by vacuum suction, as disclosed in Jpn. Appln. KOKAI Publication Nos. 61-288339 and 2-72533, is suitable for the formation of a coating on only the inner surface of a sleeve. However, it is found that it is difficult in practice to form a black coating with a uniform thickness.